1. Field of the Invention
The present invention relates to a switching regulator, and more particularly to a d.c. stabilizing power supply of a high-frequency switching system.
2. Description of the Related Art
Hitherto, for obtaining a d.c. constant voltage from a commercial power supply, there have been used a variety of switching regulators. FIG. 10 shows a schematic circuit diagram of such a conventional switching regulator. An exemplary regulator shown in the figure includes at a primary side a main circuit which consists of an a.c. power supply 100, a full-wave rectifier 102, an input smoothing capacitor 104, a primary winding of a high-frequency transformer 106, and a high-frequency semiconductor switching element which is, for example, an FET 108 (a field effect transistor). A gate of the FET 108 is connected to a gate output terminal of a pulse width modulation (PWM) control circuit 111. At a secondary side, the regulator includes a main circuit which consists of a secondary winding of the high-frequency transformer 106, a rectifying diode 110, a commutating fly-wheel diode 112, a smoothing choke coil 114, and an output smoothing capacitor 116. An output terminal of the main circuit at the secondary side is connected to an output voltage detecting resistor 118 and a voltage dividing resistor 120 as well as a load circuit of a load 122. A divided voltage between the output voltage detecting resistor 118 and the voltage dividing resistor 120 is connected to one input terminal of the pulse width modulation control circuit 111.
In the switching regulator of this type, the a.c. voltage supplied from the a.c. power supply 100 is full-wave rectified by the full-wave rectifier 102 and then smoothed by the input smoothing capacitor 104, thus causing a d.c. voltage containing a ripple component shown in FIG. 11(a) to be generated. The d.c. voltage is switched to a high-frequency pulse voltage by the FET 108, and transformed into a required voltage by the high-frequency transformer 106. The high-frequency pulse voltage thus transformed is smoothed through the rectifying diode 110, the commutating fly-wheel diode 112, the smoothing choke coil 114, and the output smoothing capacitor 116, resulting in a d.c. voltage shown in FIG. 12.
If the a.c. input voltage and a load are kept constant, then the pulse width of the high-frequency pulse voltage is constant so that a d.c. constant voltage V.sub.0 is always applied to the load. However, since the output voltage V.sub.0 is to be changed with fluctuation of the a.c. input voltage or load, the pulse width modulation control circuit 111 changes a signal to be outputted to the gate of the FET 108 in accordance with a voltage change .DELTA. V detected from the divided voltage between the output voltage detecting resistor 118 and the voltage dividing resistor 120, so that the pulse width of the primary side high-frequency pulse voltage is so controlled as to maintain the output voltage V.sub.0 constant.
However, in the above-mentioned conventional switching regulator, the d.c. voltage containing the ripple components shown in FIG. 11(a) is applied across the input smoothing capacitor 104, and current is concentrated for charging of the ripple components. As a result, the a.c. input current becomes non-linear waveforms containing a lot of odd harmonics such as third and fifth ones as shown in FIG. 11(b). For that reason, the propagation of the switching regulators of this type causes recent problems such as high-frequency faults, for example, that the transformers at transforming stations in input distribution lines are heated or abnormal sounds are produced. Also, there arises a problem such that an increased amount of reactive current components flows due to leading power factor, resulting in an increase in wiring capacity. Moreover, since the primary-side input smoothing capacitor 104 smoothes the a.c. input voltage of a low frequency, the capacity is increased, the device is large-sized and the costs are increased. Such a conventional regulator has a limit of the power factor being 60 to 70% at maximum.
To solve the above problems, the inventor has proposed an improved switching regulator from which the primary-side smoothing capacitor 104 is omitted. The switching regulator is constituted so that the high-frequency pulse voltage which has been full-wave rectified is applied from the primary side to the secondary side and a d.c. voltage is generated by a smoothing circuit provided at the secondary side. The configuration is to improve the power factor with omission of the primary-side smoothing capacitor 104 shown in FIG. 10.
However, even with such an improved regulator by the inventors, there yet remain the problems that the output smoothing capacitor 104 must be made as small as possible and also that the ripple components occur in the d.c. voltage at the output side as the capacitor 104 is made small. That is, in the case of a large load, as shown in FIG. 13, the ripple voltage appears in synchronism with a time at which the a.c. input voltage becomes 0 V. The ripple voltage cannot be reduced without increase of the capacity of the output smoothing capacitor, and even with such an improved regulator, the power factor is 85% at maximum.